Age-hardening iron-base alloy with improved toughness

ABSTRACT

A MARAGING STEEL ALLOY HAVING, IN THE AGED CONDITION, A YIELD STRENGTH IN THE 200,000 TO 300,000 P.S.I. RANGE AND A SUPERIOR TOUGHNESS, EXHIBITING CHARPY V-NOTCH VALUES IN EXCESS OF 100 FT-LBS. AT 80*F. THE ALLOY ESSENTIALLY CONTAINS LESS THAN 0.005% EACH OF CARBON AND SULFUR, 1520% NICKELT, 5-10% COBALT, 3-8% MOLYBDENUM, UP TO 1.5% TITANIUM, UP TO 0.5% ALUMINUM, AND THE BALANCE IRON WITH NORMAL INCIDENTAL IMPURITIES. IN ANOTHER EMBODIMENT, TOUGHNESS CAN BE INCREASED APPRECIABLY WTH AN INSIGNIFICANT SACRIFICE IN STRENGTH BY SUBSITUTING TUNGSTEN FOR MOLYBDENUM.

3,723,196 AGE-HARDENING IRON-BASE ALLOY WITH INEPROVED TOUGHNESS William J. Murphy and Gerald J. Spaeder, Monroeville Borough, Pa., assignors to United States Steel Corporation No Drawing. Filed June 18, 1970, Ser. No. 47,611 Int. Cl. C22c 39/10, 39/50 US. Cl. 148-31 Claims .ABSTRACT OF THE DISCLOSURE A maraging steel alloy having, in the aged condition, a yield strength in the 200,000 to 300,000 p.s.i. range and a superior toughness, exhibiting Charpy V-notch values in excess of 100 ft.-lbs. at 80 F. The alloy essentially contains less than 0.005% each of carbon and sulfur, 15- 20% nickel, 5-l0% cobalt, 38% molybdenum, up to 1.5% titanium, up to 0.5% aluminum, and the balance iron with normal incidental impurities. Hn another embodiment, toughness can be increased appreciably with an insignificant sacrifice in strength by subsituting tungsten for molybdenum.

BACKGROUND OF THE INVENTION The prior art is well familiar with a certain group of low-carbon, high-nickel martensitic wrought alloy steels known as maraging steels. Unlike iron-carbon martensite, these iron-nickel martensitic alloys are relatively soft and ductile in the quenched condition, and became strong, hard and tough when tempered or aged. These alloys are therefore readily formable and weldable, and yet can be aged to exhibit yield strengths up to 300,000 p.s.i. and at the same time provide a relatively high degree of ductility and toughness. In addition to these physical properties, the maraging steel alloys have an exceptional resistance to stress-corrosion cracking, and can be easily welded in the high-strength aged condition.

The first maraging steel used commercially contained about 0.01% carbon, 20 or 25% nickel, and 1.5 to 2.5% titanium and 0.3% aluminum. The more recent improved maraging steels however contain 0.01 to 0.03% carbon, about 18% nickel, 7 to 8% cobalt, about 5% molybdenum, and less than 1% titanium. The latter is disclosed in US. Pat. 3,093,519, R. F. Decker et al., June 11, 1963.

Although the 18% nickel maraging steel does provide a superior combination of all the desirable physical properties, there has been continuing eiforts to improve this alloy, particularly its impact toughness.

SUMMARY OF THE INVENTION In accordance with the present invention, there is provided a new and improved maraging steel alloy which will exhibit a heretofore unattainable combination of physical properties. Specifically, the alloy of this invention can be aged to produce a greater degree of toughness, impact toughness and ductility with little sacrifice in strength and other desired properties. The alloy of this invention is therefore comparable in all respects to the prior art 18% nickel maraging steel except that it provides a substantially greater degree of toughness. Broadly, this improved maraging steel is substantially like the prior art 18% nickel maraging steel chemically except that carbon and sulfur must be critically controlled to levels below 0.005% each. In accordance with another embodiment, toughness can be increased appreciably without a significant sacrifice in strength by partially or completely substituting tungsten for the molybdenum in amounts of from 3 to 8%.

Since the commercial supply of molybdenum has been rather limited at times in the past several years, the second embodiment provides the added commercial and United States Patent. Office 3,723,1'96 Patented Mar. 27, 1973 economic advantage of substituting more readily available tungsten for molybdenum.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In its broad aspect, the maraging steel of this invention has a composition, in weight percentages, as follows:

1 Balance with incidental impurities.

As noted, the above composition differs essentially from the prior art 18% nickel maraging steel in critically limiting the carbon and sulfur contents to less than 0.005 Although improvement in the steels toughness is realized within the above composition range, it can be further optimized by completely substituting tungsten for the molybdenum. In addition to restricting the residuals carbon and sulfur, the alloy additives are preferably restricted to about 8.0% tungsten or molybdenum, about 18% nickel and about 10% cobalt as a compromise between optimum physical properties and economy.

In order to derive the benefits of this invention, it is essential that carbon as well as sulfur must be limited to amounts below a critical 0.005 each. The fact that sulfur is detrimental in the 18% nickel maraging steel has been documented in the prior art. Specifically, Novak and Diran, What Are the Effects of Residual Elements in Maraging Steels? Journal of Metals, March 1963, pp. 200-204, reported that sulfur will reduce the toughness of the maraging steel substantially in proportion to its concentration. They therefore conclude that maraging steels should contain less than 0.010% sulfur and preferably less than 0.007 or 0.005%. Carbon, on the other hand, in amounts up to 0.3%, has not been considered detrimental in the prior art maraging steels and, in fact, is reported to be beneficial in the 18% nickel maraging steel in amounts from 0.01 to 0.03%

Although molybdenum and tungsten are equivalent alloying agents for some alloying purposes, tungsten has never been considered as a comparable substitute for molybdenum in maraging steels. On the contrary, a critical co-action between molybdenum and cobalt has always been deemed essential to achieve the superior properties characteristic of the newer improved maraging steels. Although pior art investigators have reported that tungsten in amounts up to 1.15% has essentially no eifect on maraging steels, in larger concentrations tungsten is known to aifect to the aging reaction and to increase strength, but at the expense of reducing toughness appreciably. As an example of such prior art teaching, see C. J.

' Novak and L. M. Diran, supra.

In contrast to the above prior art teachings, we have found that restricting carbon as well as sulfur to levels below 0.005% will improve the steels toughness; and further that the substitution of tungsten for molybdenum in the 18% nickel maraging steel will not reduce toughness of the wrought and aged product, but will in fact substantially enhance toughness if the carbon content and sulfur content is maintained below 0.005 each. At the prior art carbon levels of 0.010 to 0.030%, tungsten in amounts exceeding 1.15% does indeed reduce the products toughness. However, when carbon is below-0.005%, the combined effect of tungsten and low carbon will have an unexpected effect of increasing toughness. To appreciate this benefit, it is of course further essential that sulfur, like carbon, must be below 0.005%.

On an atomic weight basis, it would normally be expected that twice as much tungsten would be required to replace a given weight of molybdenum and attain a comparable strength level. Therefore, even if one should anticipate substitution of tungsten for molybdenum in a conventional 250-grade maraging steel having 3 to 6% molybdenum, it would require the use of 6 to 12% tungsten to effect a comparable job. Contrary thereto, the alloy of this invention contains only 3 to 8 weight percent tungsten, an amount equal to almost half the atomic weight percent molybdenum in the 250-grade maraging steel. For optimum results, i.e., optimum toughness with only minimal sacrifice in strength, the higher concentrations of tungsten are preferred, i.e., about 7 to 8%.

As in prior art maraging steels, elements frequently referred to as auxiliary hardeners, e.g. titanium, aluminum, columbium and vanadium, may be added to the alloy to cooperate with the tungsten or molybdenum in providing strength. If present in excessive amounts, hoW- ever, these elements may over-harden the alloy, thereby appreciably reducing ductility. Nevertheless, inclusion of these additives, particularly about 0.6 to 1.5% titanium and about 0.5% aluminum is preferable because optimum physical properties can be achieved at lower tungsten contents of about To more graphically illustrate the advantages of this invention, a few of our tests, and the results thereof, are exemplified below.

In one test, two maraging steels were compared. Sam- TABLE I.LONGITUDINAL:MECHANICAL PROPERTIES OF SAMPLES'IMO, 2W AND 3W It is readily seen from the'abovetable that the mere substitution of tungsten for molybdenu fiiin a conventional 250-grade maraging steel did not improve the mechanical properties. Yield strengths and'tensile strengths were comparable, but toughness in the two; tungsten containing samples was appreciably lower as expected.-

In contrast to Table I above, the physical properties of the alloys of this invention are shown in .Tables'II and III below. Table II shows the physical properties of a 250-grade maraging steel having carbon and-sulfur contents below 0.005% in accordance with the first embodiment of this invention- The samples in Table II all had substantially the same composition approximately as follows: 0.004% carbon, 0.02% manganese, 0.001% phosphorus, 0.0015% sulfur, 0.02% silicon, 17.5% nickel, 8% cobalt, 0.4% titanium, 0.08% aluminum and 4.8% molybdenum. The samples shown in Table III'were'identical to those shown in Table II except that they contained 5.1% tungsten instead of 4.8% molybdenum in accordance with the second embodiment 'of thisinvention. Except for variable annealing times as noted in the tables, all samples were processed identically.

TABLE II Yield 1 Annealstrength Elonga- Reduc- CVN ing time (02% Tensile tion in 2 tion in Energy at 900 F., offset), strength, inches, area, at 80 F., Sample hours k. s.i. k. s.i. percent percent ft.-lb.

4M0 0 107 145 1 19. 5 83.9 N.D

1 229 239 p 0 67. 5 86, 82 3 246 257 I2. 0 60. 5 65, 77, 67 8 262 269 I2. 5 60. 5 47, 3 16 263 272 12. 0 58. 8 36, 30

See footnotes at end of Table III.

TABLE III Yield Annealstrength Elonga- Reduc- CVN ing time (.02 Tensile tion in 2 tion in Ener at 900 F ofiset), strength, inches, area, at 80 Sample hours k. s.i. k. s.i. percent percent it.-

5W 0 110 136 1 19.0 84. 6 N.D'. 1 200 207 15. 0 68. 7 120,124 3 216 221 14. 5 68. 6 109,112,127 8 226 232, 14. 0 V l 68. 0 105,103 16 226 236 14. 0 68. 0 97, 89

1 Elongation in 1 inch (percent). NorE.-N.D.=Not Determined.

prior art practices and aged for 3 hours at 900 F. Table I below gives the resulting mechanical properties for each sample.

Comparing Table II with Table L- it' is readily seen that the low carbon content, as taught herein, does substantially increase toughness, with little' effect on strength. With a three hour anneal "at 900 F.," the low carbon samples had CVN values of 65, 77 and 67, as compared to 20 and 21 for the prior-art steel. v

Comparing Table III with Table'sI and II, it is'se'en that the combination of low carbon and tungsten in place of molybdenum had an even greater eifect in improving toughness. The three hour anneal samplesat 900 F, had CVN values of 109, 112 and 127. I

We claim: I 1. A maraging steel alloy consistingfessentially of 3 to 8% molybdenum, 15 to 20% nickelfS to 10% cobalt, less than 0.005% carbon, less than 0.005% sulfur and the balance essentially iron plus incidental impurities in conventional residual amounts, said steel alloy, in the aged martensitic condition, characterized by a yield strength within the range 200 to 300 k.s.i. and a Oharpy V-Notch Energy at 80 F. above 30 ft. lbs.

2. The maraging steel alloy of claim 1 containing about 8% molybdenum, about 18% nickel and about cobalt.

3. The maraging steel alloy of claim 1 further containing up to 1.5% titanium and up to 0.5% aluminum.

4. A maraging steel alloy consisting essentially of 3 to 8% tungsten, to nickel, 5 to 10% cobalt, less than 0.005% carbon, less than 0.005% sulfur and the balance essentially iron plus incidental impurities in conventional residual amounts, said steel alloy, in the aged martensitic condition, characterized by a yield strength within the range 200 to 300 k.s.i. and a Charpy V-Notch Energy at 80 F. above ft. lbs.

5. The maraging steel alloy of claim 4 containing about 8% tungsten, about 18% nickel and about 10% cobalt.

6. The maraging steel alloy of claim 4 further containing up to 1.5% titanium and up to 0.5% aluminum.

7. The maraging steel alloy of claim 6 containing about 6% tungsten.

References Cited UNITED STATES PATENTS 6/1963 Decker 148-3l l/ 1970 Decker --l23 OTHER REFERENCES C. I. Novak and L. M. Diran, What Are the Effects of Residual Elements in Maraging Steels, Journal of Metals, March 1963, pp. 200-204.

R. F. Decker, J. T. Eash, and A. I. Goldman, 18% Nickel Maraging Steel, A.S.M. Trans., 1962, vol. 55, pp. 58-76; 1010-1015.

L. DEWAYNE RUTLEDGE, Primary Examiner I. E. LEGRU, Assistant Examiner US. Cl. X.R.

75--123 J, 123 K, 123 M 

